1. METHODS References INTRODUCTION Cummins, J. (1991). Language development and language learning. In L. Malave & G. Duquette (Eds.), Language culture and cognition: A collection of studies in first and second language acquisition (pp. 161-175). Clevedon: Multilingual Matters Ltd This study involved participants from two different language study departments (Indonesian dept and English dept) and two different years of study (first year and fourth year students in the two departments) in an Indonesian private university. The participants were asked to write both in L1 and L2 argumentative essays except for the Indonesian department students who wrote only in Indonesian. The essays both in Indonesian and English would be analyzed in terms of their macro level communication processes (i.e. dialogic and monologic process) and overall qualities. Both descriptive and inferential statistics would be employed in analyzing the participants’ L1 and L2 essays. The participants were also given questionnaires in regards to their background writing instruction, L1 and L2 proficiency, and their writing experiences in general. The results of the questionnaire would be correlated to the participants’ patterns of their writing rhetorical structures. Two raters were employed to analyze both Indonesian essays and English essays and their inter-reliability scores would be measured. Many studies about cross-linguistic transfer of writing skills were approached from cultural differences perspective; therefore, developmental factors and individual differences have been given little attention. A better understanding of L2 to L1 theoretically would provide a better conception of how L1 and L2 knowledge are interrelated and how they could influence bilinguals’ L1 and L2 writing products. As indicated by the previous research (e.g. Arsyad, 1999 and Cahyono, 2000) within Indonesian context, there seems to be differences in the ways ideas organized and developed between English and Indonesian. Practically, it would also offer a basis to approach and evaluate better L2 writing teaching practice. Answers to the students’ L2 writing problems might need to be sought not only from the practice of L2 writing instruction itself but from L1 writing instruction practice as well. The Linguistic Interdependence Model (Cummins, 1991: 167) Surface features of L2 Surface features of L1 Common Underlying Proficiency In terms of conceptual literacy knowledge, Cummins (1991) argues that there is an interdependence relationship between first (L1) language and second (L2) language. Literacy knowledge that someone learns from either language can be a foundation to develop knowledge in the other language. However, most studies either supporting or challenging this theoretical perspective are from the results of research investigating a one way interaction or transfer that is from L1 to L2. Few studies have been conducted to investigate the backward transfer from L2 to L1. Many of these studies were conducted in the American and European context and there is an issue about the cognateness of the languages being studied. These studies were also done in the context of bilingual education where the L2 is the dominant language and L1 is the minority language. OBJECTIVE To investigate the possibility of L2 → L1 transfer of writing skills (rhetorical structure/argument structure) as a result of L2 and L2 writing instruction from the perspective of the interdependence system of bilinguals’ conceptual knowledge as suggested by Cummins within Indonesian context especially among students who have already received extensive L2 (English) and L2 writing instruction. To know whether language developmental factors in L1 and L2 such as L2 proficiency, L1 and L2 writing skills, and writing experiences in general also affect the patterns of the transfer. Arsyad, S. (1999). The Indonesian and English argument structure: A cross- cultural rhetoric of argumentative texts. Australian Review of Applied Linguistics, 22(2), 85-102. Cahyono, B. Y. (2000). Rhetorical strategies in the English and Indonesian persuasive essays of Indonesian university students. Unpublished Master Thesis, Concordia University, Montreal, Quebec.